Acid Base Extraction

Organic Chemistry with Vernier © Vernier Software & Technology 1

Separation of Organic Compounds by Acid-Base Extraction Techniques

A commonly used method of separating a mixture of organic compounds is known as liquid-liquid extraction. Most reactions of organic compounds require extraction at some stage of product purification. In this experiment you will use extraction techniques to separate a mixture of an organic acid, a base, and a neutral compound.

Organic acids and bases can be separated from each other and from neutral compounds by extraction using aqueous solutions of different pH values. Most organic carboxylic acids are insoluble or slightly soluble in water, but these compounds are highly soluble in dilute aqueous sodium hydroxide because the acid is deprotonated by the base producing the sodium carboxylate salt.

RCO2H(solv) + OH–(aq) → RCO2–(aq) + H2O(aq)

The carboxylic acid can be selectively isolated by dissolving the mixture in an organic solvent that is immiscible with water, and then extracting the solution with sodium hydroxide. The basic aqueous solution containing the carboxylate salt is acidified, causing the sodium carboxylate salt to convert back to the carboxylic acid, which is not water soluble. The acid will precipitate from the solution, as shown here.

RCO2–(aq) + H+(aq) → RCO2H(s)

Organic bases (e.g., amines) that are insoluble in water can be separated by extraction with hydrochloric acid. Addition of HCl to the amine produces the corresponding ammonium salt, which is soluble in water but not in organic solvents.

RNH2(solv) + H+(aq) → RNH3+(aq)

The amine can be recovered from the aqueous solution by treatment with a base, converting the ammonium salt back to the amine. The amine is not water-soluble and will precipitate, as shown here.

RNH3+(aq) + OH–(aq) → RNH2(s) + H2O(aq)

Using your understanding of these properties, separation of a mixture containing a carboxylic acid, an amine, and a neutral compound can be carried out via sequential acid and base extractions. The precipitates will be collected and characterized by melting temperature analysis.

OBJECTIVES

In this experiment, you will

Separate a mixture containing benzoic acid, 3-nitroaniline, and naphthalene. Calculate the percent recovery of each component in the mixture. Measure the melting temperature of each isolated compound.

2 Organic Chemistry with Vernier

MATERIALS

Part I Extraction

60 mL separatory funnel balancefour 50 mL Erlenmeyer flasks weighing papertwo 100 mL beakers sample mixturering stand with filtering funnel and ring diethyl etherfilter paper 6.0 M hydrochloric acid solutionring stand with utility clamp 1.0 M sodium hydroxide solutionBüchner funnel 6.0 M sodium hydroxide solutionfiltering flask sodium sulfate, Na2SO4, anhydrousspatula cold distilled water in a wash bottledisposable Pasteur pipets and bulb saturated sodium chloride solution10 mL graduated cylinder icetwo watch glasses compressed airpH paper

Part II Melting Temperature

LabQuest or computer interface isolated samples from Part ILabQuest App or Logger Pro mortar and pestleVernier Melt Station benzoic acid (optional)glass capillary tubes, one end closed 3-nitroaniline (optional)tissues (preferably lint-free) naphthalene (optional)

PROCEDURE

Part I Extraction

1. Obtain and wear goggles. Protect your arms and hands by wearing a long-sleeve lab coat and gloves. Conduct this reaction in a fume hood.

2. Weigh out approximately 1.0 g of the sample mixture. Record the mass to the nearest 0.01 g. Transfer the mixture to a 100 mL beaker and dissolve it in 15 mL of diethyl ether. CAUTION: Diethyl ether is flammable. Be sure that there are no open flames in the room during the experiment.

3. Clamp the support ring onto a ring stand and place the separatory funnel into the ring. Pour the solution into the separatory funnel and add 5 mL of 6.0 M hydrochloric acid. CAUTION: Handle the hydrochloric acid with care. Can cause painful burns if it comes in contact with the skin.

4. Cap the funnel and gently shake several times, venting frequently to avoid pressure build-up. When venting the funnel, point the tip away from your face and open the stopcock to release the pressure. Place the funnel on a support ring with a clamp and allow the solvent and aqueous layer to separate. Leave the funnel uncapped.

5. Drain the lower aqueous layer into a 50 mL Erlenmeyer flask. Repeat the extraction with another 5 mL of 6.0 M hydrochloric acid, draining the second aqueous layer into the same Erlenmeyer flask. Save the solvent layer in the separatory funnel for later use.

Organic Chemistry with Vernier 3


6. Cool the flask containing the acidic aqueous extracts into an ice water bath. Slowly add 6.0 M sodium hydroxide with a pipet until the aqueous layer is basic. Use pH paper to test. CAUTION: Sodium hydroxide solution is caustic. Avoid spilling it on your skin or clothing.

7. Set up a vacuum filtration.

a. Weigh the filter paper and record the mass to the nearest 0.01 g.b. Place a Büchner funnel and filter paper on the filtration flask.

c. Collect the precipitate by vacuum filtration. Wash the filtrate with 2 mL of cold distilled water. d. Place the filter paper containing the solid on a watch glass and gently direct a stream of air (low flow) to thoroughly dry the solid. e. Weigh the recovered solid and record the mass to the nearest 0.01 g.f. Save the solid for the melting temperature analysis in Part II.

8. Extract the saved ether layer in the separatory funnel with three 5 mL portions of 1.0 M sodium hydroxide. Drain the aqueous layer into a 50 mL Erlenmeyer flask. Save the ether layer in the separatory funnel.

9. Cool the flask containing the basic aqueous extract in an ice water bath. Using a pipet, slowly add 6.0 M hydrochloric acid until the aqueous layer is acidic. Use pH paper to test. CAUTION: Handle the hydrochloric acid with care. Can cause painful burns if it comes in contact with the skin.

10. Set up a vacuum filtration.

a. Weigh the filter paper and record the mass to the nearest 0.01 g.b. Place a clean Büchner funnel and filter paper on the filtration flask.c. Collect the precipitate by vacuum filtration. Wash the filtrate with 2 mL of cold distilled

water. d. Place the filter paper containing the solid on a watch glass and gently direct a stream of

air (low flow) to thoroughly dry the solid. e. Weigh the recovered solid and record the mass to the nearest 0.01 g.f. Save the solid for a melting temperature analysis.

11. Add 10 mL of saturated aqueous sodium chloride solution to the ether solution remaining in the separatory funnel and shake gently. Be sure to vent frequently.

12. Allow the layers to separate and discard the lower aqueous sodium chloride layer. Pour the solvent layer into an Erlenmeyer flask containing approximately 1 g of anhydrous sodium sulfate and allow it to stand for about 10 minutes, swirling occasionally.

13. Set up a gravity filtration.

a. Place a filtering funnel into a ring on a ring stand.b. Weigh the filter paper and record the mass to the nearest 0.01 g. Place the filter paper

into the funnel. c. Filter the contents in the flask, including the sodium sulfate, into a clean 100 mL beaker. d. Direct a gentle stream of air at the beaker to evaporate the ether. As the ether evaporates,

a solid will begin to crystallize in the beaker. CAUTION: Diethyl ether is flammable. Be sure that there are no open flames in the room during the experiment. Do not leave unattended.


14. Weigh the recovered solid and record the mass to the nearest 0.01 g. Save the solid for the melting temperature analysis in Part II.


15. Obtain a small amount of the isolated solid from the acid extraction. The solid should be in a powdered form. If it is not, use a mortar and pestle to carefully grind the solid to a powder.

16. Prepare a sample for melting.

a. Pack a capillary tube 3–4 mm (~1/8 inch) deep with your sample by inserting the open end into a small pile of the solid. A small amount of the solid will be pushed up into the tube.

b. Tap the closed end of the capillary tube on the table top to compress the sample into the closed end.

c. Check the control knob on the Melt Station to confirm that it is in the Off position.d. Carefully insert the capillary tube of solid into one of the sample holders of the Melt

Station.

17. Connect the Melt Station power supply to a powered electrical outlet.

18. Connect the Melt Station sensor cable to LabQuest or to a computer interface.

19. Start the data-collection program, then choose New from the File menu. You are now set up to take melting temperature data for up to 20 minutes.

20. In the first trial, you will want to observe the melting process and make a rough estimate of the melting temperature of your sample. Don’t worry if the heating rate is a bit too rapid, and the sample melts too quickly. To do this:

a. Start data collection.b. On the Melt Station, turn the control knob to a setting of 180ºC. The red light will turn on

indicating active heating.c. Carefully observe your sample. If the solid begins to melt, click Mark to mark the

temperature on your graph (or press the D key on the computer or the OK button on LabQuest). When the entire solid has completely melted, click Mark again. The two values marked on your graph describe the estimated melting temperature range of your substance.

d. If the solid does not melt by the time the temperature gets to 150ºC, turn the control knob to the 220ºC setting. Continue observing your sample, and if the sample begins to melt, mark the temperatures on the graph as previously described.

e. If the sample has not melted by the time the temperature gets to 190ºC, turn the knob to the Rapid Heat setting. When the sample finally begins to melt, mark the graph as previously indicated.

f. When you have determined the approximate melting temperature range for the sample, stop data collection. Store the run by tapping the File Cabinet icon in LabQuest, or choosing Store Latest Run from the Experiment menu in Logger Pro. Discard the capillary tube and sample as directed by your instructor.

g. On the Melt Station, turn the control knob to the Fan/Cooling setting to get ready for the next trial. The blue light will turn on indicating that the fan is cooling the Melt Station.



21. Now that you have a rough idea of the melting temperature, a more accurate determination of the melting temperature can be made. Use a previously prepared sample in a capillary tube, as described in Steps 15–16, to determine the melting temperature of the sample:

a. Start data collection.b. On the Melt Station, turn the control knob to the Rapid Heat setting.c. Carefully observe the temperature vs. time graph. When the temperature is within

approximately 10ºC of the lowest possible melting temperature of your sample, turn the control knob to a temperature setting corresponding to your expected melting temperature.

d. Carefully observe your sample. When the solid begins to melt, click Mark to mark the temperature on your graph. When the entire solid has completely melted, click Mark again. The two values marked on your graph describe the estimated melting temperature range of your substance. When you are finished with this step, stop data collection.

e. Store the run. f. Discard the capillary tube and sample as directed by your instructor.g. On the Melt Station, turn the control knob to the Fan/Cooling setting to get ready for the

next trial.

22. Repeat Step 21 until you have determined the melting temperature range of the solid. Record the range in the data table.

23. Repeat Steps 20–21 for the isolated solids from the base and neutral extractions.

24. At the end of the experiment, turn the control knob on the Melt Station to Off.

DATA TABLE

Part I Extraction

Mass of mixture (g)

Mass of sample recovered (g)

Benzoic acid

3-Nitroaniline

Napthalene


Measured melting temperature range (C)

Benzoic acid

3-Nitroaniline

Napthalene


DATA ANALYSIS

1. Draw the structure of each of the compounds.

2. Outline a flow chart describing the separation of the mixture and the isolation of each compound.

3. What was your percent recovery for each of the three compounds? Assume equal amounts of the carboxylic acid, amine, and neutral compound were present in the unknown mixture.



INSTRUCTOR INFORMATION

1. The sample mixture can be prepared with 1:1:1 benzoic acid, 3-nitroaniline, and naphthalene by mass.

2. The melting temperature ranges of benzoic acid, 3-nitroaniline, and naphthalene are 121–123°C, 111–114°C, and 80–82°C, respectively. Note: If you want to shorten the amount of time students spend determining these melting temperatures, you could let them know these three expected melting temperatures ahead of time.

3. Prepare the 6.0 M hydrochloric acid solution by adding 493 mL of concentrated HCl to distilled water for 1 L of solution. Note: Add acid to water, never water to acid.

4. Prepare the 1.0 M sodium hydroxide solution by adding 40 g of solid NaOH to enough distilled water for 1 L of solution.

5. Prepare the 6.0 M sodium hydroxide solution by adding 240 g of solid NaOH to enough distilled water for 1 L of solution. CAUTION: Heat is released. The reaction is exothermic.

6. Prepare a saturated aqueous sodium chloride solution by adding NaCl in distilled water until it no longer dissolves and a shallow layer of NaCl is visible at the bottom of the bottle.

7. The filter paper can either be fluted or folded in quarters for gravity filtration.

8. Dry each sample thoroughly before taking a melting temperature.

9. The level of peroxide formation in diethyl ether can vary depending on the length of exposure to air, light, and the container type. Periodically test for peroxides and do not store for a long period of time.

10. Dispose of hazardous waste properly.

HAZARD ALERTS

Hydrochloric acid: Highly toxic by ingestion or inhalation; severely corrosive to skin and eyes; wear gloves and eye protection when using this substance. HMIS Classification: Health hazard3, Flammability0 Physical hazard0.

Sodium Hydroxide: Corrosive solid; skin burns are possible; much heat evolves when added to water; very dangerous to eyes; wear gloves and eye protection when using this substance. HMIS Classification: Health hazard3, Flammability0, Physical hazard2.

Benzoic acid: Slightly toxic by ingestion; body tissue irritant; combustible. HMIS Classification: Health hazard2, Flammability1, Physical hazard0.

Diethyl ether: Serious fire hazard (flash point 40.0°C). Irritating to body tissues. Moderately toxic by ingestion. Vapor causes weakness, fatigue, nausea and headache. Skin and eye irritant. HMIS Classification: Health hazard2, Flammability4, Physical hazard2.

3-Nitroaniline: May cause respiratory tract irritation. Skin and eye irritant. Moderately toxic by ingestion. HMIS Classification: Health hazard2, Flammability1, Physical hazard0.


Naphthalene: Toxic by inhalation and ingestion. Skin and eye irritant. May be harmful if absorbed through the skin. HMIS Classification: Health hazard3, Flammability0, Physical hazard2.

Sodium sulfate: May cause respiratory tract irritation. Skin and eye irritant. Moderately toxic by ingestion. HMIS Classification: Health hazard0, Flammability0, Physical hazard0.

Sodium chloride: May cause respiratory tract irritation. May cause skin and eye irritation. HMIS Classification: Health hazard1, Flammability0, Physical hazard0.

The hazard information reference is: Sigma-Aldrich Co., 1-800-325-3010, www.sigmaaldrich.com/safety-center/msds-search.html

COMPOUND INFORMATION

Compound Chemical formulaMelting temperature

range (C)Molar mass

(g/mol)

benzoic acid C7H6O2 121–125 122.1

3-nitroaniline C6H6N2O2 111–114 138.1

naphthalene C10H8 80–82 128.1

sodium sulfate Na2SO4 884 142.0

sodium hydroxide NaOH 318 40.0

sodium chloride NaCl 801 58.4

Compound Chemical formula Molar mass (g/mol) Density (g/mL) at 25°C

diethyl ether C4H10O 74.1 0.706

Compound Chemical formula Concentration (%) Density (g/mL) at 25°C

hydrochloric acid HCl 37 1.12



SAMPLE RESULTS

Melting temperature graph of benzoic acid.

Melting temperature graph of 3-nitroaniline.


Melting temperature graph of naphthalene.

Documents

Acid Base Extraction